1. Field of the Invention
The present invention relates to a capacitive probe for measuring the level of an electrically conductive liquid in a vessel, particularly suited for high pressure and high temperature applications, and a method of manufacturing such a probe.
2. Description of the Prior Art
Examples of well-known methods intended to measure the evolution of the level of a liquid in a vessel are optical methods according to which the progress of a float is monitored by automatic control through a transparent window provided vertically in the wall. This type of measurement is suited only for clean liquids which do not affect the transparency of the window.
There are also ultrasonic methods where the level of the liquid in a vessel is measured by measuring the time of propagation of pulses reflecting on the surface. This type of measurement works well even at relatively high pressures and temperatures as long as the interface between phases is clear, but it loses precision when the liquid is highly emulsified.
It is also well-known to detect the position of the interface between an electrically conductive liquid by means of a capacitive probe whose capacitance is affected by the level variation.
Such a probe comprises for example a central metallic rod sheathed with an insulating plastic material, applied for example by heat shrinkage, which dips into the conducting fluid. The variation of the inter-electrode capacitance between the sheathed rod and the conducting fluid, resulting from the level variation, is measured. A probe of this type is for example used in the device which is described in French Patent 2,772,477 of the Assignee.
There are also level-measuring capacitive probes made up of a first electrode consisting of the metallic wall of a vessel which is separated from the conducting liquid by an insulating layer made of, for example, Teflon(trademark). It can be the wall of a tube made of an insulating plastic covering the inner metallic wall of the vessel or an insulating coating (enamel for example) evenly deposited on the inner face. The inter-electrode surface is here the surface of the inner wall of tube 1. A probe of this type is used for example in the device described in French Patent 2,782,804 of the Assignee.
The capacitive probes mentioned above however have some drawbacks. First of all, it is difficult to deposit a uniform and thin insulating layer on a metallic rod or wall. The probes generally work properly under normal temperature and pressure conditions, but they deteriorate very quickly in more difficult environments notably as a result of creep effects affecting the plastic sheaths. This degradation occurs even when the insulating sheaths or coatings are made of materials withstanding heat, such as enamels for example. Enamel contains bubbles and thermal stresses cause microcracks that are invaded by the conducting liquid. The dielectric constant of the sheath or of the coating can thus vary considerably according to whether the sheath is more or less saturated with the liquid, which of course distorts the measurement results.
The capacitive probe according to the invention allows measurement of the level of a conducting fluid in a vessel with good precision, measurements being stable in time and reproducible, which allows overcoming the aforementioned drawbacks of the prior art.
The probe comprises a tubular element made from an electrically insulating material, with a first face in contact with the conducting fluid forming a first electrode, and an opposite face in contact with a second electrode, and a means for measuring the capacitance variations of the capacitor formed by the electrodes, the second electrode being made of a fusible light alloy deposited on the opposite face of the tubular element made of insulating material, whose melting temperature is selected according to the specified maximum temperature range within which the capacitive probe is intended to work.
For applications in a range of temperatures up to 200xc2x0 C. for example, a lead/tin type alloy whose melting point is of the order of 300xc2x0 C. can for example be selected.
The second electrode can be formed by filling the inner space of the tubular element with a fusible alloy whose melting temperature is selected according to the capacitive probe, or it can be cast between the inner face of the tubular element and a central core.
This central core can in some cases consist of a solid rod. The central core can however also have the shape of a tubular element made from the same heat-resisting material. Two capacitors fitted into each other can thus be formed by placing a third electrode into this hollow core made of heat-resisting material. As the capacitors are exposed to the same temperature, the inner capacitor can be used as a reference and thus allows compensation for the effects of temperature on the dielectric permittivity of the heat resisting material and therefore to compensate for the concomitant variations of the capacitive probe.
According to another embodiment, the insulating layer is annular and arranged in the vessel. It electrically insulates the wall of the vessel from the electrically conducting liquid contained therein. Similarly, the second electrode can be a coating or a layer of fusible metal cast between the heat-resisting layer and the outer wall.
Various metal oxides can be used as the heat-resisting material, such as zirconia or alumina which, unlike enamels, are practically insensitive to chemical and biological attack, and stable regarding pressure and temperature variations.
It can be easily checked that the capacitive probe according to the invention is well-suited to work within a wide temperature and pressure variation range because of the stability of each insulating layer as regards these parameters, and also because of the probe""s perfect resistance to various corrodants, both chemical and biological.
The method of manufacturing the capacitive probe as defined above comprises making the insulating layer by baking a heat-resisting material such as a metal oxide (zirconia, alumina or others) and forming a metallic electrode obtained by fusion of a fusible alloy at an intermediate temperature between the baking temperature of the insulating layer and the maximum operating temperature of the capacitive probe.